The invention relates generally to liquid gauging systems of the type that use ultrasonic echo ranging to determine liquid levels. More particularly, the invention relates to methods and apparatus for discriminating true and false echoes to improve accuracy of such systems.
It is well known to use ultrasonic echo ranging to determine liquid levels. Common applications include fuel gauging systems in fuel tanks. Typically, one or more ultrasonic transducers are disposed near the bottom of a liquid tank or container. The transducers emit ultrasonic pulses on the order of 1 megahertz frequency towards the liquid surface. Each ultrasonic pulse is reflected at the fuel/air interface and returns in the form of an echo pulse. The echo pulses are then detected by the same transducer that transmitted the pulse or are detected by a different sensor. The detection sensor typically produces an electrical output signal that corresponds to receipt of the echo. Thus, the round trip time from pulse emission to echo detection corresponds to the distance of the liquid surface from the sensors. Characterization data of the fuel tank can thus be used with the level detection data to determine liquid quantity in the tank.
Ultrasonic liquid level detection in fuel tanks such as are used on aircraft is complicated by several factors. First, water tends to accumulate in the bottom of the fuel tanks, particularly on aircraft that fly at higher altitudes over extended distances, such as, for example, transoceanic commercial flights. Water at the bottom of the tank can present a fuel/water interface that reflects ultrasonic energy in the form of false echoes back to the transducers when such transducers are disposed at or near the tank bottom. Such false echoes can be mistaken for true fuel level echoes and thus give a false indication of fuel level and quantity.
Another problem that arises in fuel tanks is the presence of air bubbles. Aircraft manufacturers have run tests that indicate the presence of air bubbles, under some conditions large in size and quantity, due to fuel slosh and vibration under various flight scenarios. Air bubbles present a fuel/air interface that can reflect ultrasonic energy in the form of false echoes. These echoes can also be misinterpreted as false liquid level readings.
Accordingly, the objective exists for apparatus and methods for discriminating true and false echoes in ultrasonic liquid level sensing systems. Such apparatus and methods should be capable of distinguishing true echoes from false echoes such as may be caused by air bubbles and other false interfaces.